Most cable network systems are coaxial-based broadband access systems that may take the form of all-coax network systems, hybrid fiber coax (HFC) network systems, or RF over glass (RFOG) network systems. Cable network system designs, including, for example, cable television (CATV) network system designs, typically use a tree-and-branch architecture that permits bi-directional data transmission, including Internet Protocol (IP) traffic between the cable system head-end and customer locations. There is a forward or downstream signal path (from the cable system head-end to the customer location) and a return or upstream signal path (from the customer location back to the cable system head-end). The upstream and the downstream signals occupy separate frequency bands. In the United States, the frequency range of the upstream band is from 5 MHz to 42 MHz, 5 MHz to 65 MHz, 5 MHz to 85 MHz, or 5 MHz to 200 MHz, while the downstream frequency band is positioned in a range above the upstream frequency band.
Customer locations may include, for example, cable network subscriber's premises. Typical signals coming from a subscriber's premises include, for example, set top box DVR/On Demand requests, test equipment data channels, and Internet Protocol output cable modem carriers defined by the Data Over Cable Service Interface Specification (“DOCSIS”), which is one communication standard for bidirectional data transport over a cable network system.
Because the subscriber's premises is not the property of the cable network operator and signals from many subscribers' premises on a particular node combine in the return band, the return path/upstream band of the cable network system is subject to significant noise, with every subscriber's premises acting like an antenna. Noise (ingress) that intrudes into the upstream band, which is typically called ingress, accumulates along the tree-and-branch architecture, as in a funnel, until it reaches the cable head-end. Sources of ingress include short wave radio signals, broadband noise generated by, for example, personal computers and electric motors, and intermittent noise, including impulse noise, which are very short bursts of broadband noise (e.g., ranging in duration of one nanosecond to a few tens of microseconds).
Technicians have typically used signal level meters to measure ingress. This is usually done by physically connecting the signal level meter to a test point in the CATV system and using spectral analysis and/or heat maps to locate noise in the return band spectrum.